Polarimetric optical device with an insulating layer between detectors

ABSTRACT

This optical polarimetric detector comprises: 
     a first active detector element having a photoconductor ( 1 ) with which a first diffraction grating ( 3 ) is associated allowing incident light from a first specific polarization direction to be coupled in the first detector element and allowing the latter to detect the light having this first polarization direction; 
     a second active detector element having a photoconductor ( 1 ′) with which a second diffraction grating ( 3 ′) is associated allowing incident light from a second polarization direction to be coupled in the second detector element and allowing the latter to detect the light having this second polarization direction. 
     Furthermore, a detector ( 2 ) is provided making it possible to eliminate the background noise detected by previous detectors.

The invention relates to an optical polarimetric detector making itpossible to detect two polarizations and, at the same time, eliminatingthe background noise.

The object of the present invention is to describe the architecturemaking it possible to combine two specific functionalities alreadydemonstrated by quantum well detectors; the possibility of producingbispectral devices (French patent No. 2 756 667) and the possibility ofproducing detectors integrating, in the active layer, the function ofsubtracting the continuous component, that is to say the dark current ofthermal origin and the optical scene current (French patent No. 2 756666). Each of these two functions requires a double stack of quantumwells and three stage connector technology. The invention makes itpossible to keep this number of stacks and of connector technologystages, while making it possible to read in subtractive mode andaccording to three, vertical (V), horizontal (H) and mixed (V+H)polarimetric templates.

The invention therefore relates to a bifunctional optical detectorcomprising:

a first active detector element having a photoconductor with which afirst diffraction grating is associated allowing incident light from afirst specific polarization direction to be coupled in the firstdetector element and allowing the latter to detect the light having thisfirst polarization direction;

a second active detector element having a photoconductor with which asecond diffraction grating is associated allowing incident light from asecond polarization direction to be coupled in the second detectorelement and allowing the latter to detect the light having this secondpolarization direction.

The invention also relates to a method of producing such a detector.

The various objects and features of the invention will become apparenton reading the description and the appended figures:

FIG. 1, an exemplary structure of a detector according to the invention;

FIG. 2, a top view of the structure of FIG. 1;

FIG. 3, a matrix of detectors according to the invention;

FIG. 4, a table of operation modes.

The proposed architecture is identical to that of a subtractivecomponent. The spectral response of a quantum well detector is theconvolution of the spectral absorption of the detector and of thespectral efficiency induced by the geometric resonance of thediffraction grating used for the optical coupling of the incidentradiation. Thus, a one-dimensional diffraction grating resonating aroundλ and oriented in a specific direction (or in a perpendicular direction)coupled to the quantum structure described above will induce an opticalresponse centered around λ and sensitive to the vertically (orhorizontally) polarized incident light.

The principle of the invention is to separate the main pixel of size a×ainto two subpixels of size a×a/2, each of these subpixels being aconventional subtractive structure, but having two different couplingarrays (FIGS. 1 and 2).

The bias of the lower stage Vref, common to the set of pixels remainscommon to both types of subpixels. On the other hand, the biased of theupper stage, which is also common to the set of pixels, must be dividedin two, each subarray of subpixels being biased in a common manner. Theconnection technology of the upper stage is, in this case, provided bytwo bias lines V₁ and V₂ per pixel instead of a single line for astandard subtractive pixel, as will be seen in FIG. 4.

The device may then be used in three subtractive modes corresponding tothe three different polarization modes V, H and H+V, depending on thebias voltages which are applied to each of the upper electrodes (FIG.4).

According to a preferred embodiment of the invention, an opticaldetector is produced on a transparent substrate through which theincident light reaches the detector. An optical detector of this typetherefore comprises, on the substrate, a structure such as shown in FIG.1. This structure comprises, on one face of the substrate, a first ohmiccontact layer 5, this ohmic contact layer being transparent. A stack oflayers 2 forming a stack of quantum wells capable of detecting at leastone range of wavelengths is located on this ohmic contact layer. Asecond transparent ohmic contact layer 4 is on this stack of layers. Twoidentical or almost identical stacks of layers 1, 1′, each one forming astack of quantum wells capable of detecting at least one range ofwavelengths λ, are on the second ohmic contact layer and finally,diffraction gratings 3 and 3′ having different structures are on each ofthese stacks of layers. Each of these gratings allow either the lightpolarized in one direction (for example vertical) or the light polarizedperpendicularly (horizontally) to be coupled in the stack of layers (1,1′ on which the grating is produced). To do this, the two gratings areoriented perpendicularly (see FIG. 2).

As shown in FIG. 1, the assembly is encapsulated in an insulating layer6 and connection means pass through this insulating layer in order toreach the diffraction gratings and produce connection means C1 and C′1enabling voltages +V1 and +V2 to be applied respectively to thediffraction gratings 3 and 3′. Also, at least one connection meanspasses through the insulating layer and reaches the second ohmic contactlayer 4 so as to produce the common contact Cc allowing thephotoconduction signal to be read. And finally, a connection means C2 isproduced on the first ohmic contact layer 5 so that the voltage −Vrefcan be applied to the assembly of the structure.

FIG. 2 shows, in top view, a pair of detector elements like that of FIG.1. In the exemplary embodiment of FIG. 1, the stack of quantum welllayers 2 in fact forms two elementary detectors, each of them beingassociated with a detector element 1 or 1′ for the purpose ofeliminating the background noise as was explained in French patent No. 2756 666.

The stacks of quantum well layers 1, 1′, 2 are active or photoconductingat the same ranges of wavelengths λ. The stacks of layers 1 and 1′ aredesigned so as to be more absorbing at wavelengths λ than the stack oflayer 2. The stack of layer 2 is preferably designed so as to bevirtually nonabsorbing. This may be achieved by different thicknesses ofthe stacks of layers or by greater doping of the quantum well layers ofthe more absorbing stacks.

Since the detector is illuminated by incident radiation on the ohmiccontact layer 5, the stack of layers 2 receives the radiation first. Thelatter therefore passes through the stack of layers 2 then the stacks oflayers 1 and 1′ and they are coupled by the diffraction gratings 3 and3′, depending on the polarization of the light in the stacks of layers 1and 1′.

Although the diffraction grating 3 is provided in order to couple thevertically polarized light, the vertically polarized light will bediffracted toward the stack of layer 1 and will be absorbed or almostabsorbed by this stack of layers. Similarly, although the diffractiongrating 3′ is provided in order to diffract horizontally polarizedlight, the horizontally polarized light will be absorbed or almostabsorbed by the stack of layer 1′.

FIG. 3 shows a matrix embodiment of a detector according to theinvention. In this figure, four optical detectors as described abovehave been shown by way of example. Each detector therefore comprises twodetector elements, each one with its diffraction grating. In thisfigure, the connection means making it possible to control, in a matrixmanner, this matrix of detectors is essentially shown. It can thereforebe seen in this FIG. 3 that the connection means C1 are interconnectedby their contact P1 to the control potential V1. Similarly, theconnection means C′1 are interconnected by their connection contact P′1to the same control potential V2. The common contact layer, to which acontact P2 is connected, is connected to the reference potential Vref.Finally, in the central part of each detector, a connection means Cc islocated making it possible to connect an individual current measuringmeans for each detector.

In FIG. 3, a control circuit CU is also shown making it possible todeliver the reference potential Vref and the control potentials V1 andV2.

A detection circuit DET receives information v1.2 indicating the controlmode delivered by the circuit CU (potentials V1 and/or V2) from thecontrol circuit CU. It also receives read signals delivered by eachpixel to its common contact means Cc.

FIG. 4 gives a control table for detectors produced in this way. Thistable thus illustrates the control of the detectors by the circuit CU.As can be seen in this FIG. 4, in operating mode, a reference potentialVref is applied to the various detectors. When a potential V1 is appliedto one detector and no potential V2 is applied, this detector makes itpossible to detect horizontally polarized light of wavelength λ.Conversely, if a potential V2 is applied but no potential V1 is appliedto a detector, the latter detects the vertically polarized light (ofwavelength λ). Finally, if both potentials V1 and V2 are applied to adetector, this makes it possible to detect both polarizations.

The normal operating mode may be considered as being the third mode, inorder to acquire the maximum signal. The alternating use of modes 1 and2 allows a polarimetry function to be added. Finally, in the event ofoptical countermeasure attack of one of the two polarizations, it isnoted that the system can continue to operate and to acquire the signalin the other polarization.

An example of a method for producing a detector according to theinvention will now be described. To simplify the description, theproduction of a detector comprising two detector elements such as thedetector of FIG. 1 will be considered, but the application of thismethod could relate to producing a matrix of detectors.

This method comprises the following steps:

A first ohmic contact layer 5, transparent to the wavelengths to bedetected, is produced on a transparent substrate (not shown in FIG. 1).

A stack of quantum wells able to be photoconducting at a range ofwavelengths λ is produced on this ohmic contact layer.

A second ohmic contact layer, transparent to the wavelengths to bedetected, is then produced.

A second stack of quantum wells, able to be photoconducting at the rangeof wavelengths λ, is produced on this second ohmic contact layer.

Two diffraction gratings, oriented perpendicularly in order to diffractlight to the second stack of quantum wells, each for differentpolarizations, are produced on this second stack.

The geometry of each detector is then defined by etching. To do this,the assembly of layers is etched to the first ohmic contact layer 5. Agroove is also produced between the two detector elements, at theboundary of the two gratings 3 and 3′, so that the stacks of layers 1and 1′ of the two detector elements are also separated.

An insulating encapsulation layer 6 is deposited on the assembly thusobtained.

Finally, the connection means are produced. To do this, the followingholes are made:

(a) the holes passing through the insulating layer to the diffractiongratings 3 and 3′ which are metalized in order to form the connectionmeans C1 and C′1;

(b) the common connection hole passing through the insulating layer andthe second stack to the second ohmic contact layer in order to form thecommon connection means Cc allowing [lacuna] the read signal;

(c) a hole passing through the insulating layer and reaching the firstohmic contact layer 5 so as to produce the connection means C2.

It should be noted that, in the foregoing, the diffraction gratings 3and 3′ may be metalized in order to apply the potentials V1 and V2.

Thus, the method of the invention has made it possible to produce thedetector of FIG. 1. In order to produce a matrix of detectors, it wouldbe appropriate to etch several detectors designed in this way on thefirst ohmic contact layer.

What is claimed is:
 1. An optical polarimetric detector comprising: afirst active detector element having a photoconductor with which a firstdiffraction grating is associated allowing incident light from a firstpolarization direction to be coupled in the first detector element andallowing the first detector element to detect the incident light havingthe first polarization direction; and a second active detector elementhaving a photoconductor with which a second diffraction grating isassociated allowing incident light from a second polarization directionto be coupled in the second detector element and allowing the seconddetector to detect the incident light having the second polarizationdirection.
 2. The optical detector as claimed in claim 1, wherein thefirst diffraction grating is oriented to couple, in the first detectorelement, the incident light polarized in the first polarizationdirection and the second diffraction grating is oriented to couple, inthe second detector, the incident light polarized in the secondpolarization direction.
 3. The optical detector as claimed in claim 1,wherein the first and second detector elements are similar and allow asame range of wavelengths to be detected.
 4. The optical detector asclaimed in claim 1, wherein the first and second detector elements areproduced in a same layer of a photoconducting material.
 5. The opticaldetector as claimed in claim 4, wherein the first and second detectorelements are produced in stacks of layers forming quantum wells.
 6. Theoptical detector as claimed in claim 1, wherein the first detectorelement is associated with a third detector element separated by one ofa common contact layer and an insulating layer; a fourth detectorelement also being associated with the second detector element andseparated from the second detector element by a contact layer or aninsulating layer; the first, second, third, and fourth detector elementsbeing photoconducting under effect of a same range of wavelengths. 7.The optical detector as claimed in claim 6, wherein the common contactlayer or insulating layer form a same layer, and wherein the thirddetector element and the fourth detector element also form a samedetector element.
 8. The optical detector as claimed in claim 6, whereinthe first, second, third, and fourth detector elements enable the sameranges of wavelengths to be detected.
 9. The optical detector as claimedin claim 8, wherein a response of the first and second detector elementsis greater than a response of the third and fourth detector elementssuch that the third and fourth detector elements absorb light energy ata wavelength range for which they are photoconducting.
 10. The opticaldetector as claimed in claim 9, wherein the first and second diffractiongratings are made of a conducting material or are coated by a conductingmaterial and each diffraction grating comprises a first and a secondcontact means, and wherein the optical detector further comprises: athird contact means connected to faces of any of the first, second,third, and fourth detector elements that are in contact with the commoncontact or insulating layer; fourth contact means in contact with thethird and fourth detector elements on faces away from the common contactor insulating layer; means for applying control voltages to the first,second, and fourth contact means; and current conduction measuring meansconnected to the third contact means for measuring photoconduction ofthe detector elements.
 11. The optical detector as claimed in claim 10,further comprising control means for making possible applying areference voltage to the fourth contact means together with: either afirst control voltage to the first contact means to control operation ofthe first and third detector elements; or a second control voltage tothe second contact means to control operation of the second and fourthdetector elements; or both first and second control voltages to controloperation of the first, second, third, and fourth detector elements. 12.The optical detector as claimed in calm 11, further comprising a matrixof detector elements, the fourth contact means being common to all thedetector elements to be able to apply a voltage to all the detectorelements of the matrix, all the first contact means being connectedtogether to apply, on demand, the first control voltage to all the firstdetector elements of the matrix, all the second contact means beingconnected together to be able to apply, on demand, the second controlvoltage to all the second detector elements of the matrix.
 13. Theoptical polarimetric detector of claim 1 made by a method comprising:producing an ohmic contact layer on a face of a transparent substrate;producing a first stack of layers forming a stack of quantum wellsallowing a range of wavelengths to be detected, on said ohmic contactlayer; producing a second ohmic contact layer on the first stack oflayers; producing a second stack of layers forming a stack of quantumwells allowing a range of wavelengths to be detected, on the secondohmic contact layer; producing layers of at least two diffractiongratings having different physical structures, on a surface of thesecond stack of layers; etching at least two detector elements in anassembly of layers obtained until reaching the first ohmic contactlayer, a first of the at least two detector elements having adiffraction grating oriented in a first direction and a second of the atleast two detector elements having a diffraction grating oriented in adirection perpendicular to the first direction; encapsulating theassembly in an insulating layer; etching first holes in the insulatinglayer, which pass through the insulating layer and reach the diffractiongratings, and metalizing the first holes to produce the first and secondcontact means; producing at least one second hole per pair of detectorelements, said at least one second hole reaching the second ohmiccontact layer, and metalizing the at least one second hole to producethe third contact means; producing a third hole passing through theinsulator and reaching the first ohmic contact layer and metalizing thethird hole to produce the fourth contact means.